There are many choices of protecting groups for alcohols, phenols, carbonyls, carboxylates, thiols, and amines, but few examples of protecting groups for sulfonic acids.1
Protection of sulfonates as simple esters is problematic because sulfonate esters are potent electrophiles. To overcome this issue, a number of sterically-hindered protecting groups for sulfonic acids have been proposed and utilized for the synthesis of sulfonated molecules. Secondary isopropyl (iPr) sulfonates react more slowly with nucleophiles, but are poorly stable to acidic conditions, chromatography, and prolonged storage.2,3,4
Isobutyl (iBu) sulfonates are more stable to acidic conditions and can be stored, but exhibit increased sensitivity to nucleophilic cleavage.2,5
Neopentyl (Neo) sulfonates are highly hindered and thus strongly resistant to nucleophilic displacement, but are difficult to remove.6
Trichloroethyl (TCE) sulfonates are stable to non-basic nucleophiles, but react with basic nucleophiles.7
Triggered “safety-catch” sulfonate protecting groups have been described that utilize the inherent stability of neopentyl sulfonates, combined with an intramolecular trigger that allows selective removal.6,8
Roberts et al.6
pioneered this approach, creating a Boc-containing neopentyl protecting group dubbed “Neo N-B”; removal of the Boc group with TFA followed by subsequent neutralization of the unmasked amine allows cyclative cleavage of the sulfonate. However, these triggered sulfonate protecting groups have not found wide use, in part due to the need for their multi-step synthesis, and inherent high cost. Ideally, protecting groups should be inexpensive, stable to a wide variety of conditions, and selectively cleaved to liberate the free sulfonate without requiring further purification steps.
There has been little direct comparison of the intrinsic stability properties of sulfonate esters formed from commercially-available alcohols to reaction conditions commonly encountered in organic synthesis. Since my lab routinely synthesizes sulfonated molecules, we are interested in expanding the range of available sulfonate protecting groups and in establishing the chemical stability of each sulfonate ester.
Beta-fluorinated electrophiles are particularly resistant to nucleophilic substitution due to electronic deactivation of reactivity.9
For example, trifluoroethyl iodide and trifluoroethyl sulfonates are highly recalcitrant to nucleophilic substitution; displacement requires high temperatures and extended reaction times.10
Potential sulfonate protecting groups thus include esters of difluoroethanol (DFE), hexafluoroisopropanol (HFIP), trifluoroethanol (TFE), and alpha-(trifluoromethyl)benzyl alcohol (TFMB).11
Other candidate sulfonate protecting groups include phenyl (Ph; because it is sp2
-hybridized), tetrahydropyran-2-methyl (THPM; reported to be more stable to nucleophiles than iBu12
), and 3-methyl-3-oxetane-methanol, which is nominally a neopentyl alcohol ().
Commercially available alcohols as potential sulfonate protecting groups.
Dansyl sulfonate esters of twelve candidate alcohols were synthesized to screen the stability of each protecting group to different reaction conditions. Dansyl esters fluoresce yellow-green, while the liberated free dansyl sulfonate fluoresces blue. This allows the rapid detection of cleavage by TLC and by visual inspection of the reaction vial using a hand-held UV lamp. The results are shown in .
Stability profiles of dansylates formed from commercially-available alcohols.
Sodium iodide in refluxing acetone is a non-basic nucleophile that deprotected nBu, iBu, iPr, oxetane, THPM, and DFE esters. While THPM and DFE showed greater stability to cleavage among this group, they nonetheless succumbed under conditions commonly used in the Finkelstein reaction. In contrast, Neo, TFE, TCE, Ph, HFIP and TFMB were inert.
Piperidine (20% solution in DMF) is a basic nucleophile that readily cleaved the nBu ester, and reacted with the TCE ester to form a sulfonamide. The TCE sulfonate ester has been previously described to be labile to nucleophilic amines such as piperidine, and to be prone to formation of dichlorovinyl esters when used as a protecting group for sulfates.7
Prolonged treatment with piperidine (overnight, rt) also resulted in the complete cleavage of iPr, iBu, DFE, and oxetane, and gave partial cleavage of THPM. HFIP yielded a complex mixture of products that were not identified. Only Neo, TFE, Ph and TFMB survived intact.
Neo sulfonates are known to be deprotected with small nucleophiles at high temperature, such as overnight treatment with tetramethylammonium chloride in DMF at 160°C.6
Milder cleavage of Neo sulfates has been reported with a slight excess of NaN3
in DMF at 70°C.12
Under similar conditions in DMSO, TFMB was cleaved and partial cleavage of Neo and TFE sulfonates was observed (). HFIP rapidly reacted to form a side-product, presumably the sulfonyl azide. TCE was more stable, but yielded the same side-product. Heating to 100°C completely cleaved Neo and TFE; only Ph was inert to these conditions.
Treatment with NaOH under non-aqueous conditions in 9:1 DCM/MeOH13
at room temperature cleaved HFIP and TCE in under an hour. Interestingly, HFIP underwent a very rapid transesterification to the methyl ester prior to hydrolysis. Neo esters were stable to these conditions, while Ph, TFMB, and even TFE sulfonate esters were cleaved after overnight incubation at room temperature. Cleavage of TFE sulfates
has previously been reported to require refluxing with potassium t-butoxide in t-butanol.14
All of the sulfonate esters evaluated in this work were stable to mildly reducing conditions such as NaBH4
. TCE sulfonates have been reported to be cleaved by reduction with zinc,7
and in this study TCE was the only sulfonate found to be removed by reduction with iron ().
Most sulfonates are stable to moderately acidic conditions. Only the iPr ester was found to be labile to TFA at rt for 16h. The lability of the iPr group is presumably due to formation of a stabilized secondary carbocation.
Hot strong acids cleave most sulfonates. Even Neo has been reported to be labile to overnight reflux in 6M HCl.7
Cleavage under these conditions is presumably due to methyl migration.15
In this study, it was found that refluxing in 48% HBr for two hours also cleaved Neo, as well as iPr, nBu, iBu, THPM, and TFMB. DFE was partially cleaved under these conditions, while oxetane 12
reacted with HBr to open the oxetane ring, but interestingly did not cleave to the sulfonate. Only TCE, Ph, HFIP and TFE survived intact. Similarly, treatment with concentrated sulfuric acid at room temperature for 90 minutes cleaved Neo and TFMB dansylates, but not TCE, HFIP, TFE or Ph.
Screening of Lewis acids revealed that the Neo group can be removed under even milder conditions. The Lewis acid BBr3, commonly used to cleave aryl methyl ethers, was found to rapidly remove Neo in less than 15 min at 0°C. Oxetane 12 formed the ring-opened brominated product, as was observed in HBr. These conditions also removed TFMB, but not TCE, HFIP, DFE, TFE, or Ph.
In some circumstances, Neo has been reported to be cleaved under less acidic solvolysis conditions. For example, Liu et al.16
have found that Neo protection of a difluoro-sulfotyrosine residue within a peptide can be removed by extended (4–5 day) treatment with 0.1% TFA. In this case, the fluorinated sulfonate is expected to increase the rate of solvolysis. Similarly, Simpson et al.17
have recently reported that Neo sulfates
in peptides can be cleaved by treatment with ammonium acetate (2M, 37°C, 6h). However, these conditions had no effect on Neo dansylate, even at 60°C, possibly due to poor solubility. When dissolved in DMSO, diluted with 2M ammonium acetate, and heated at 100°C for 2h, only partial cleavage was effected.
Six esters are stable to sodium iodide: Neo, TFE, TCE, Ph, TFMB, and HFIP (). To examine their stability to other reaction conditions on a preparative scale, the respective p-toluenesulfonyl esters (tosylates) were prepared.
Treatment with 20% piperidine in DMF is well-tolerated by TFE, Ph, Neo, and TFMB tosylates 13
(). On the other hand, the TCE ester 17
reacts to form p-toluenesulfonyl piperidine (TsPip),7
and the HFIP ester 18
gives a complicated mixture of products.
Stability of p-toluenesulfonate esters (tosylates).
Treatment at room temperature with 2 eq of NaOH in 9:1 DCM/MeOH cleaves most of the tosylates; only Neo survives (). This deprotection method is particularly useful, as the precipitated sulfonate can be easily separated by filtration and/or extraction. In the case of 16, simple filtration afforded pure sodium p-toluenesulfonate (NaOTs). For 13, the filtered product was contaminated with sodium trifluoroethoxide, but could be purified by subsequent acidification and removal of the trifluoroethanol. Alternatively, extraction rather than filtration affords pure NaOTs.
Conversely, treatment with 3eq of BBr3
at 0°C cleaves both Neo and TFMB tosylates, but leaves TFE, Ph, HFIP and TCE tosylates unaffected. Complete cleavage of 15
could also be achieved with 1 eq of BBr3
at −78°C. No neopentyl bromide or alcohol was recovered, suggesting that methyl migration occurred during the deprotection.15
Replacement of BBr3 with the milder Lewis acid BCl3 was equally effective, allowing the isolation of NaOTs in 92% yield after treatment of 15 with 1 eq of BCl3 for 30 minutes at 0°C.
Overall, Neo, TFE, and Ph groups are the most broadly stable sulfonate protecting groups. Ph exhibits the highest stability to nucleophiles, even hot NaN3
. TFE and Ph are cleaved under basic conditions, while Neo is complementary in its stability as it is cleaved by hot aqueous acid or strong Lewis acid treatment (). TFMB sulfonates can be cleaved under acidic or basic conditions, yet exhibit high stability to most nucleophiles. TCE and HFIP sulfonates are poorly stable and reactive under basic conditions, but are highly stable to iodide and acidic conditions. TCE esters are also uniquely labile to reducing conditions ().3
These screening results have established the intrinsic lability of sulfonate esters based on commercially-available alcohols, and can serve as a guide for the judicious selection of a sulfonate protecting group. Moreover, two mild cleavage conditions have been described that together cleave virtually all sulfonate protecting groups, at or below room temperature. Most sulfonates, including TFE and Ph, can be cleaved at room temperature with NaOH under non-aqueous conditions. Sulfonates that are prone to solvolysis in hot protic acid, such as Neo and TFMB, can be cleaved with a stoichiometric amount of BBr3 or BCl3, well below room temperature. Finally, the general stability of fluorinated sulfonate protecting groups suggests that, like the neopentyl group, they are suitable platforms for the construction of protecting groups with engineered lability.